Add prefix `v1` to tracker API endpoint

[josecelano]

The tracker has added a prefix to the API endpoints with the version:

torrust/torrust-tracker#239

So API URLs have changed from (without version):

http://0.0.0.0:1212/api/stats?token=MyAccessToken

To (with version on the path v1):

http://0.0.0.0:1212/api/v1/stats?token=MyAccessToken

We have to add the prefix in the TrackerService.

For the time being, the tracker also keeps the old URLs. Once this issue is finished, we have to open a new issue on the tracker repo to remove the support for API URLs without the version prefix.

[josecelano]

Hi @da2ce7 @WarmBeer I think in the long term, we could extract the API client into an independent package or as a feature in an API package. But for the time being, I'm going to just add the prefix v1.